ISOTHERMAL MICROCALORIMETRY AS A TOOL TO PROBE PARASITIC REACTIONS IN LITHIUM-ION CELLS

Abstract

Understanding the mechanisms affecting the lifetime of lithium-ion cells is critical to extend cell lifetime and increase the energy density for applications such as grid energy storage and electric vehicles. Unwanted, or parasitic, reactions between the electrode materials and the electrolyte can consume the available lithium and electrolyte components, limiting cell lifetime. This work developed methods of probing parasitic reactions in-situ and non-destructively in short experiments using isothermal microcalorimetry. The effect of different electrode materials, electrolyte additives, and solvents on cell lifetime was investigating by isolating the heat flow due to parasitic reactions – the parasitic heat flow. Three methods were developed in this work. The first method probed the reactions occurring during the formation of the solid electrolyte interphase, which occurs the first time a cell is charged. The measured heat flow was compared to theoretical estimates of the heat flow of proposed reaction pathways using computed values in the literature. The second method used the measured heat flow during slow charge-discharge cycles to isolate the parasitic heat flow as a function of cell voltage. The method was used in multiple studies to explore the effects of electrolyte additives, solvents, positive electrode coatings, positive electrode composition, and negative electrode materials on the parasitic heat flow. The technique correlated with cell performance in almost all cases, but the results suggested that differences between electrode or electrolyte components could introduce different types of parasitic reactions, affecting the parasitic heat flow. Finally, a method was introduced to probe the reaction enthalpies of parasitic reactions using high-precision voltage-holds. The study found that different solvents and positive electrode coatings played a large role in determining the types of parasitic reactions occurring in cells. Additionally, the rates and types of parasitic reactions were found to change significantly with the cell voltage. The methods developed in this work provided insight into the mechanisms responsible for limited lifetime in the cells studied and contributed to a better understanding of parasitic reactions in lithium-ion cells. New measurement techniques found that changes in parasitic processes could be quantified, and depend on cell voltage, electrode materials, electrode coatings, and electrolyte chemistries.